I would probably go with netting since you're already having issues. I agree that a few bushes or small trees would be helpful as well.
I have a similar open 30' x 50' run that my girls "free range" in when I'm home, the rest of the time they're in a predator proof run. They love to hang out under the sumacs. I also planted a cedar tree and an apple tree.
I have gotten lucky though. A few months ago I was outside and looked up to see a bald eagle up high. I thought it was after a smaller bird, but it turns out a couple smaller birds were dive bombing to chase it away.
A few weeks ago I was inside and heard some crows causing a ruckus. I came out to see about 10 of them swarming a hawk. They called in reinforcements and another 10 or so showed up and the hawk left.